Flooded-houses-in-Lumberton

Florence: Queen of Floods

In News by ambrisk

Florence:
Queen of Floods

Flooded houses on the outskirts of Lumberton, North Carolina, September 17, 2018. (Reuters/Jason Miczek)

Hurricane Florence smashed into the US East Coast, leaving at least 13 people dead, knocking out power to nearly a million homes, triggering widespread flooding and landslides. The first major storm of the season was well anticipated, with over a million people evacuated ahead of landfall. We review the path and impact of the storm to-date and highlight how the flooding will leave an increasingly devastating and costly impact on industries, communities and the environment.

Tropical Depression Six formed late on Aug. 31 in Mali, Africa then was named Tropical Storm Florence the next day in the far eastern Atlantic Ocean. Within 5 days, Florence became a Category 4 hurricane after rapidly intensifying over the open Atlantic Ocean. Its strength fluctuated with winds varying between 75 and 130 mph in just 25 hours before landfall at Wrightsville Beach in North Carolina.

Rains and surging seas inundated coastal areas, with conditions deteriorating significantly on Friday 18th September, as the slow-moving Hurricane Florence crawled along at 6mph. The slow speed combined with the size of the storm system has led to the National Hurricane Centre forecasting that some 18 trillion gallons of rainwater could fall before the storm is finished with North Carolina.

Floodwater levels are still rising

The first inundation came from a huge storm surge, which reached more than 3 metres at Top Sail Beach. Similar levels were experienced in New Bern, North Carolina, where some residents stayed put despite evacuation warnings and waited to be rescued. The storm surge reversed rivers deep into the state – these water courses have now broken their banks as the record rainfall in the upper catchments meets the ingress of coastal water.

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Hurricane Florence after making landfall on Friday, Sept. 14, 2018 (NOAA / NBC News)

Heavy rain in the Appalachian mountains has triggered mud and rock slides and is a portent of the huge volume of water yet to travel down the catchment to the coast – which could take up to 2 weeks to work through.

By Monday 17th September, record breaking, still-rising floods were covering much of North Carolina, which have prevented regulators and environmental groups from making a detailed assessment.

By that afternoon, Elizabethtown had received 91 cm (or 36 inches) of rain with 50cm typical across the rest of the region. More than 30 rivers have flooded, with 15 at the “major flooding” level (equivalent to the UK’s Severe Flood Warning), according to real time gauges on the North Carolina Flood Inundation Mapping and Alert Network.

Cape Fear River reached some 19 metres (62 feet) and is not expected to drop below the 10 metre (35-foot) median flood level for another week. Meanwhile the city of Wilmington was been cut off from the outside world for some 4 days, with severe damage to the I40 Interstate motorway.

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Cape Fear River – rapid rise over three consecutive days (photo courtesy of TIME Magazine)

Assessing the environmental impact of Florence

Aside from power cuts, there are significant concerns about utility and other industrial installations which could have created enormous pollution incidents. The Duke Energy Corporation has a power plant close to Wilmington which has suffered a major landfill coal ash spill, following inundation from the hurricane rainfall.  Some 2000 cubic yards (1530 cubic metres) – 2/3rds of an Olympic sized pool – has spilled into watercourses which will now be dispersing across a wide area.  This could contain a toxic cocktail of arsenic, mercury, lead and selenium.

There are also unverified reports received by the US Environmental Protection Agency that an estimated 5 million gallons of wastewater spilled into Cape Fear from a treatment plant when two generators failed on the day the storm hit.

There are also some 4000 hog lagoons across North Carolina with highly toxic slurry, producing over 200 million gallons of waste every year. These lined earth pits have been a major environmental concern for a long time but they have held out so far – in part due to the efforts to drain them onto productive farmland well ahead of the forecast hurricane.

Analysing hurricane flood risk

While there is plenty of analysis of Florence to be undertaken, the storm emphasises how warmer seas with higher levels of moisture are powering hurricanes. While this hasn’t been the most intense, it clearly follows a trend (like Hurricane Harvey last year) of far higher rainfall, where flooding becomes the prime risk, not just structural damage to buildings.

Data from meteorological observers has shown that the annual recurrence interval for some locations exceeded the 1,000 year level, breaking previous records for the state. This follows similar unprecedented rainfall events setting new highs in four states since August 2017. Climate scientists are considering this as strong evidence that storms are getting wetter and even advise caution not to consider this as the new normal as that may undersell the situation as global warming is predicted to warm the planet more, and consequentially we may expect to see rainfall increases well beyond the present.

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Average recurrence intervals for maximum 72-hour rainfall during Florence (MetStat)

This non-stationarity of climate increases uncertainty and causes concern amongst government, academics and the insurance community. Clearly there is a heightened need to not rely on outdated data. Fortunately, Ambiental’s USA FloodMap currently in development will include all recent events in its hydrological model inputs to provide a view of flood hazard which overcomes some of the challenges of existing flood maps such as those produced by FEMA and shown to underpredict current hazard. Furthermore Ambiental’s FloodFutures modelling approach uses future facing predictive climate models to predict what flooding under a warming climate will look like, which helps insures, property and infrastructure asset managers with long term planning for resilience.

Initial estimates from reinsurance firm JLT predicted event losses to insurers of $10 – $20 billion in the lead up to Florence. Current estimates from AIR Worldwide now put this far lower at $1.7 – $4.6 billion. It is not clear which proportion of this estimate relates specifically to flood damages, and it will take some time before we know the true cost of the event. However, it is certainly apparent that the flood related event loss will be significant, and as the US hurricane season is only just starting there could be more substantial losses in the coming months.

Ambiental has been applying our forecasting, monitoring and loss modelling technologies to better understand this event. Forecasting systems such as FloodWatch enable greater alerting of floods which can improve decisions prior to and during a flood event which has potential for reducing impacts and saving lives. Working with satellite data partners we are able to detect the floods from space and produce flood extent layers which provide a better understanding of the situation on the ground. Through enabling greater understanding of how floods develop and the impacts they have using advanced modelling it is possible to make the best of this bad situation and ensure future generations have the tools they need to deal with increases in flood risk.

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In the lead up to Florence making landfall Ambiental was analysing the impacts of the event through an understanding of human and economic exposure. Impacts will vary considerably based on the exact route the storm takes.

Ambiental is developing solutions to support countries and businesses around the world to better understand flooding. Contact us to find out more or email [email protected].